1. Field of the Invention
The present invention is in the field of nondispersive infrared (NDIR) gas analysis of a type tyically used to measure the concentration of specified gases in medical breath analyzers and in medical instruments. More particularly, the present invention relates to an improved NDIR gas analyzer which accurately measures the concentration of a selected one of a plurality of gases in a multi-component gas mixture.
2. The Prior Art
The term "nondispersive" as used herein refers to the apparatus used, typically a narrow-band optical or infrared filter instead of a prism or diffraction grating, for isolating radiation in a particular wavelength band for measurement purposes. The isolated wavelength band normally coincides with a strong absorption band in the absorption spectrum of a gas to be measured.
The NDIR technique has been widely used in the gas analyzer industry for the detection of single-component and multi-component gas mixtures. Such gas analyzers utilize the principle that various gases exhibit substantial absorption charactristics at specific wavelengths in the infrared radition spectrum. Several types of these gas analyzers exist which utilize a number of arrangements of components such as the source, sample chamber, optical filter, reference cell and detector. In one such gas analyzer shown and described in U.S. Pat. No. 3,793,525 by Burch et al. the beam of infrared energy passing through the sample chamber containing the unknown gas mixture is varied by the interposition of one or more narrow band-pass filters such as on a filter wheel in the path of the infrared energy beam. Typically, each filter only passes radiation at the characteristic absorption wavelength of a particular gas of interest. Another filter may also be used as a reference filter at a wavelength close to but not overlapping the characteristic absorption wavelength of any of the gases present in the sample cell. This type of gas analyzer also requires the generation of some type of synchronizing signal in order to coordinate the operation of the signal processing circuitry with the rotation of the filter wheel.
Another type of NDIR gas analyzer is shown and described in U.S. Pat. No. 3,811,776 by Blau Jr. which incorporates in addition to the infrared source, sample chamber, narrow band-pass filter and detector, a reference cell (a gas cell containing the gas of interest, in this case CO.sub.2) and an identical cell evacuated or filled with a gas that is transparent at the wavelength used (4.26 microns for CO.sub.2). These two cells alternately are moved into and out of the radiation beam. Since a sample chamber is placed in series with these cells, the alternate introduction of the absorbing and nonabsorbing cells into the radiation beam creates a reference (absorbing or reference cell) and a sample (non-absorbing cell) detector signal whose ratio is used to determine the CO.sub.2 gas concentration in the sample chamber. Unlike the configuration described in U.S. Pat. No. 3,793,525 alluded to earlier which utilizes two interposed optical filters to create a sample and reference detector signal, the Blau configuration takes advantage of the principle of non-linear absorption by the gas to be measured (CO.sub.2) as discussed in U.S. Pat. No. 4,578,762 by Wong in order to create the reference and sample signals.
Yet another and improved type of such gas analyzer is shown and described in U.S. Pat. No. 4,694,173 by Wong. This gas analyzer has no moving parts such as a rotating wheel for effecting either the interposition of optical filters or absorbing and non-absorbing cells to create both a sample and a reference detector signal as in the NDIR gas analyzers described earlier.
All of the NDIR gas analyzers described hitherto for the measurement of the concentrations of one or more gases in a mixture assume the fundamental fact that the infrared absorption bands of these gases are respectively specific, i.e. they do not spectrally overlap in the radiation spectrum. As a matter of fact none of the above types of NDIR gas analyzers will function as intended if the gas or gases to be measured have nonspecific absorption bands in the infrared portion of the electromagnetic spectrum. Under this circumstance they will all suffer significant interferences from other gases that share a portion of their absorption bands and will be rendered unacceptably inaccurate in many or even almost all applications.
The NDIR technique has been widely accepted over the years in the gas analyzer industry primarily because the gases of interest for both medical and industrial applications typically have strong and specific absorption bands in the infrared. On the medical side the very strong and specific 4.26 micron absorption band of CO.sub.2 is directly attributable to the success and acceptance of the NDIR CO.sub.2 gas analyzer. On the industrial side such as in the area of automotive exhaust emission monitoring, the principal gases (hydrocarbons, CO and CO.sub.2) all have strong and specific absorption bands at 3.3-3.4, 4.67 and 4.26 microns, respectively.
There are instances where the gases that need to be measured simultaneously have very significantly overlapping absorption bands in the infrared. A well-known example is the case of the anesthetic halocarbons Halothane, Enflurane and Isoflurane. These three gases, being hydrocarbon derivatives, have relatively weak absorption bands bunched together in the 3.3-3.5 micron region, much like the other hydrocarbons. However, they also have strong but overlapping absorption bands beyond the 3.3-3.4 micron region extending all the way to 16 microns. Since these gases are very important to the field of medicine espcially in the practice of anaesthesia, instruments are needed for their accurate and precise measurement.
Monitors based upon several different physical principles have been available for use in measuring anesthetic halocarbons, e.g. elastomer string, coated piezoelectric crystal, refractometer, gas chromatograph, spectrophotometers operating in the ultraviolet and infrared ranges, and mass spectrometer. Except for infrared and mass spectrometers, these methods have been less than satisfactory for routine clinical use, because of deficiencies such as bulky sensors, interference by carbon dioxide, nitrous oxide and water vapor, and excessive drift of zero and gain.
Although the mass spectrometer has dominated the field of anesthetic halocarbon measurement over the years, NDIR analyzers for these gases do exist side by side with the mass spectrometer. These NDIR anesthetic halocarbon analyzers utilize both the strong 8-14 micron and the much weaker 3.3-2.5 micron absorption bands for their measurement. However, due to the overlapping characteristics of their absorption bands at these wavelengths, all halocarbons (Halothane, Enflurane, and Isoflurane) are measured at the same wavelength band defined by one appropriately chosen band-pass filter. Different gain factors are used in the signal processing circuitry to set proper sensitivity for each halocarbon as disclosed and described in U.S. Pat. No. 4,480,190 by Burough et. al. With this approach specific halocarbons cannot be automatically identified and the user must manually designate the halocarbon being measured. Furthermore these NDIR instruments show much poorer performance in terms of sensitivity, specificity and stability when compared to their counterparts for the measurement of other gases such as CO.sub.2, CO and methane. For those instruments which use the weak 3.3-3.4 micron absorption band the modulation is hardly sufficient to allow a good measurement. This leads to poor sensitivity, a bulky sample chamber and vulnerability to interference from other gases, albeit at moderate levels. Thus a need exists for an improved approach for the detection and analysis of these gases.
As expectations for the practice of safe medicine continue to grow (partly because of the increase in the number of operating room malpractice suits being filed and partly because of the general awareness of the possible disasters that an average patient may face) the need for an improved anesthetic halocarbon monitor becomes more urgent. In addition to better sensitivity, specificity and stability that are expected of these instruments, the ability to automatically identify individual halocarbons is crucial. At the least these instruments should possess the capability to inform the anesthesiologist that a different halocarbon is being administered to the patient other the one he thinks is being delivered. No presently available halocarbon monitor known to the inventors possesses this desirable feature.